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Timing irrigation for Louisiana soils

Doreen Muzzi | Jun 26, 2003

An irrigation scheduling program developed by the University of Arkansas has made timing irrigation treatments easier for many growers, and now researchers in Louisiana have tinkered with the program to make it better fit producers in their state.

It’s important to let soil types specific to the state dictate when and how much water specific crops need, says Keith Collins, Richland Parish Extension Agent in Rayville, La. Collins was a speaker at the recent LSU AgCenter’s Northeast Research Station Field Day in St. Joseph.

Because irrigation timing and the amount of water applied per irrigation should be based on the crop’s water use, the moisture content of the soil, and expected rainfall, determining moisture deficits are “critical,” he says.

“The Arkansas program recommends trigger points based on soil types and water holding capacity, and while some of our soils are very similar to those in Arkansas, we’ve had to change the recommended deficit levels for our heavier clay soils and our drought prone Macon Ridge soils,” Collins says.

“Their recommended deficits are a little high for our really drought-prone soils on the Macon Ridge, and irrigation is triggered a little early for us on narrow-row soybeans planted on our heavy clay soils.”

To accommodate the water needs of the plant, the scheduling program recommends growers irrigate when soil moisture levels reach a pre-determined level. The program indicates available soil moisture in inches of water, and then measures inches of water used from the soil by a particular crop. This determines how much water the crop is allowed to use before irrigation is recommended. The program also calculates daily water uses from crop growth stage, temperature, rainfall and irrigation data.

According to Collins, the Arkansas scheduling program triggers an irrigation treatment on silt loam soils with a hard pan whenever moisture deficits reach 1.75 inches for soybeans, or 2 inches for cotton. For those growers planting on Macon Ridge silt loam soils, he recommends reducing those deficits to 1.5 inches for both soybeans and cotton.

“I’ve found Arkansas’s recommended moisture deficits to be just about right for our deeper silt loam soils,” Collins says. “However, on our Macon Ridge silt loam soils, the pans generally run 6 to 8 inches deep, which means that’s all the area you’ve got to grow a crop in before you hit that hard pan. With everything happening in the top 8 inches of soil, plants undergo drought stress much quicker than they would in other soil types.”

On Louisiana’s heavy clay soils, Collins says they’ve raised the recommended deficit from 2 inches to 2.25 inches for those growers planting drilled, narrow-row soybeans. “Otherwise, it seems to be making us come back a little too quick with another irrigation treatment,” he says.

On the deep commerce silt loam soils along the Mississippi River, Collins says Louisiana cotton growers may be able to increase Arkansas’ recommended deficit level of 2.5 inches to 3 inches.

“More than three years of research has shown that the cotton plant’s root system can tap into that water table, which is close enough to the surface for the tap root to get into it. The rare exception is in cases of extreme drought.”

While the irrigation scheduling program accounts for all sources of water and all soil types, it does not account for poorly drained fields, or extended periods of cloudy weather.

“It assumes excellent management of your crop,” says Al Coco, research associate with the LSU AgCenter’s Northeast Research Station in St. Joseph, La. “Bear in mind, it may be thinking your crop is physiologically older than it is, especially when cloudy days or windy conditions have delayed management treatments.”

“It also requires you to water earlier than most folks do, and come back more frequently. That requires more labor than some folks may have available,” adds Collins.

Efficient irrigation water management, researchers say, can reduce moisture extremes and associated plant disease problems, which in turn may reduce the need for pesticides.

“There are several irrigation models available, but we’ve had good luck with the Arkansas scheduling program,” Collins says. “I’ve had excellent success with corn, cotton and soybeans. This is the first year, though, that I’ve worked with a producer on a more drought tolerant crop like grain sorghum. There is very little irrigated grain sorghum grown in Louisiana, and it historically has not been a serious cash crop for most of our producers.”